An inter-institutional cooperative study is being conducted to determine the effectiveness of oral administration of acetylsalicylic (aspirin) in the reduction or prevention of transient cerebral ischemic attacks and amaurosis fugax. In addition, the effect on the frequency of cerebral and myocardial infarction in long-term follow-up will be evaluated. This is a controlled clinical drug trial being conducted in accordance with very strict rules in a formalized manual of procedure. Only individuals having the hemispheric type of transient cerebral ischemic attacks or monocular blindness will be included. Those with primary vertebrobasilar insufficiency only, or with potentially fatal complicating medical conditions, will be excluded. Care is being taken to exclude individuals who are found to have blood dyscrasia or specific idiosyncrasy to aspirin. Thus far this has not constituted any significant problem. Subjects are allocated to subgroups on the basis of a clinical decision to operate or not operate. In each of the two subgroups subjects are being assigned to treatment with aspirin or placebo on the basis of a random statistical design. This design should enable the investigator to make meaningful comparisons between therapies in each subgroup. Strict surveillance is being maintained of the patient's adherence to the instructions for therapy. The administration of the active drug and placebo is being carried out on a coded double blind basis. A mechanism is provided for in the project design for breaking the code in the event of clinical necessity.